1. Field of the Invention
This invention relates to a capacitive transducer circuit and to a method of biasing a capacitive transducer, and in particular, but not exclusively, to a MEMS capacitive transducer such as a MEMS microphone.
2. Description of the Related Art
FIG. 1 shows a capacitive transducer 3 comprising a first electrode that is biased to a first bias voltage V1 supplied by a voltage source 5, and a second electrode that is connected to a node 7 biased to a second bias voltage V2′ via a high resistance 4. The voltage VoutT on node 7 is input to an amplifier 9, which outputs a corresponding voltage VoutA on an output terminal 11.
The transducer is designed so that its capacitance changes according to some input stimulus. For example, transducer 3 may be a MEMS microphone, where one electrode is fixed, and the other one moves in response to the pressure waves of incident sound. Since the capacitive transducer is arranged to drive into a high impedance on node 7, the charge on the capacitor does not change significantly. Therefore, the change in the capacitance of the transducer due to the stimulus results in a change ΔV in the voltage across the transducer capacitance.
This transducer signal component ΔV is proportional to the relative change in capacitance of the transducer 3. It is also proportional to the charge stored on the capacitor, and so is proportional to the applied bias V1-V2′. Typically ΔV is only a few millivolts for normal audio signals from a MEMS microphone, since the electrode displacement is only small and there are practical limits to the applied bias voltage across the transducer for both the sensor and the voltage supply. Typically, the bias voltage V2′ will be near ground, and the bias voltage V1 about 12V.
The voltage source 5 may be a capacitive charge pump, for example, since the bias voltage V1 may be higher than any external voltage VIN supplied to the system; it may also be other circuitry such as an inductive DC-DC converter, and it may comprise a linear regulator, perhaps fed using a bandgap reference circuit to act as a reference voltage. Each of these sources will present some thermal noise at their output. However, any noise voltage on the bias voltage V1 output from the voltage source 5 will be indistinguishable from the small transducer signal ΔV mentioned above. Thus, it is preferable that the voltage source 5 is designed to inject minimal noise in the signal band.
The DC bias V2′ of the other transducer terminal at node 7, coupled to the input of the amplifier 9, is defined by a bias voltage V2 and a series resistance 4. V2′ will usually be near to ground in order to maximise the voltage across the transducer element. It is also preferable that the circuitry providing this bias does not inject any appreciable noise into the system.
It will be appreciated from inspection of FIG. 1 that the transducer signal ΔV will appear on node 7, high-pass filtered by the transducer capacitance and the bias resistance 4. A typical MEMS microphone will have a capacitance of the order of only 1 pf. Thus, to provide a −3 dB corner frequency of 20 Hz requires a bias resistance of approximately 8 GΩ. Such a high value resistance is difficult to implement accurately as an actual resistor, and therefore the resistance 4 may be obtained by using a diode near the origin of its I-V characteristic. However, it is still difficult to define such a high resistance accurately, so the corner frequency, and possibly the start-up time, of such transducer circuitry is difficult to predict or to maintain in volume production. To guarantee a minimum value of 8 GΩ the typical value may have to be much higher, for example 25 GΩ to 80 GΩ, or even higher.
Also, it is preferable that the amplifier 9 used to pick up the signal VoutT from the capacitive transducer 3 has a higher input resistance than the (minimum) value of resistance 4, as must any additional circuitry attached to node 7.
Furthermore, the amplifier 9 must also have an input capacitance substantially lower than the transducer capacitance in order to avoid attenuating the small change in voltage ΔV generated by the movement of the membrane. Any other circuitry attached to node 7 must also present only a small capacitance.
In practical implementations of the circuit shown in FIG. 1, there may be leakage currents onto node 7. For example, a leakage current IL can flow across the terminals of the biased transducer 3. Such leakage currents IL will alter the current through resistance 4, thus creating an error in the output signal VoutT from the capacitive transducer 3, and hence the analog output signal VoutA. For example, if resistance 4 is 80 GΩ, even 100 fA of leakage will give rise to a DC offset of 8 mV, which is comparable with a normal peak audio signal. This DC offset voltage may disturb the operating point of the amplifier and thus degrade its linearity, or give an objectionable effect downstream such as reducing the available dynamic range of downstream circuitry, such as gain amplifiers or an ADC.
It will be appreciated by those skilled in the art that VoutT=V2′±ΔV where V2′ represents the DC (bias) voltage and VoutT represents the AC (signal) voltage.
An aim of the present invention is to provide a capacitive transducer circuit, and a method of biasing a capacitive transducer, which provides a predictable DC output voltage (and hence corner frequency) with low added noise.